project planning: fix #10686 regression

[alt-romes]

We previously only fetched the compiler in
instantiateProjectConfigSkeletonFetchingCompiler if null (toListOf traverseCondTreeV skel) was false.

c89ab54 changed this to always configure the compiler, such that it could be later re-used when determining whether jsem was supported to clone git submodules concurrently.

This patch allows for the JobControl concurrency mechanism to be controlled without a compiler (which assumes -jsem is not supported), and restores the behaviour of not always configuring the compiler.

Fixes #10686

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[alt-romes]

It's not clear to me how to add a test to this. I've tried adding a check for directory-does-not-exist for the scripts run in PackageTest, however, because of flags that set output directories more precisely.

@ffaf1 I've run out of time, but perhaps you could try to write a test if you'd like.

[Bodigrim]

I tested manually that the patch fixes my reproducer from #10686. Good enough for me, thanks.

[ulysses4ever]

Makes sense to me. Thanks!

[alt-romes]

I don't know why the readthedocs check failed @ulysses4ever

[ulysses4ever]

@alt-romes it started recently and no one knows why. The branch needs an update to trigger rebuild as we can't trigger RTD rebuild manually, sadly.

Also, "no rebase" labels are reserved for special cases. Normally, you're expected to use "merge me".

[Mikolaj]

@mergify rebase

[mergify]

rebase

✅ Branch has been successfully rebased

[Mikolaj]

Phew, readthedocs passes fine now. Thank you whomever fixed it. :)